1. Field of the Invention
This invention relates to fluent abrading processes and apparatus. More particularly, this invention relates to an improved mixing or focusing tube for a high speed, abrasive, fluid jet cutting apparatus.
2. Description of the Related Art
Cutting with water is a well-known technology that has been prevalent since the 1970's. Water jet cutting is one of a number of technologies known as power beams. These include laser cutting, plasma arc cutting and oxy-acetylene gas cutting.
By utilizing a high-pressure pump to pressurize water to ultra high pressures and then forcing the water to flow through a tiny orifice can result in water jets that have velocities that are up to three times the velocity of sound. Such a focused water jet has sufficient kinetic energy to cut through most hard-to-cut materials, and when abrasives are mixed with the water flow so as to yield an abrasive water jet, one can efficiently cut almost any type of material.
Because of their greater cutting power, abrasive water jets account for nearly 60% of the water jet cutting market. Typical applications include the cutting tasks associated with fabrication of structures using extremely hard materials, such as titanium and the super-alloys, and in various mining and drilling applications where hard rocks must be cut. Meanwhile, plain water jets are used for industrial cleaning, surface preparation and paint stripping applications, and for the cutting of food products, paper and plastic materials, and woven (e.g., carpet) and nonwoven (e.g., filtration materials) products. Saline, water cutting jets have also been used in medical applications.
The primary equipment associated with a typical, abrasive water jet cutting system is shown in FIG. 1. It consists of an incoming water treatment system, a booster pump for optimal operation of downstream filters, an intensifier pump that raises the water's pressure to ultrahigh levels, high pressure plumbing that delivers the ultrahigh pressure water to the system's cutting head, an abrasive feeder system that supplies the abrasive particles that are mixed with the ultrahigh pressure water in the cutting head, and an outgoing water catcher and treatment system.
The typical cutting head for an abrasive water jet is shown in FIG. 2. A sapphire, diamond or ruby orifice is used as the initial orifice to create a high velocity water jet. The typical diameter of such orifices is 0.07-0.7 mm. A dry abrasive, such as garnet, silica or alumina (with typical particle sizes being 125-180 microns), is aspirated/entrained into the mixing chamber by the vacuum created by the water jet. It mixes with the water jet and the mixed slurry jet is then collimated by a mixing tube (also called a focusing tube) before exiting the cutting head through the mixing tube's exit orifice. The diameters of the passages through such mixing tube are 0.5-3 mm, with tube lengths of 50-150 mm.
The most troublesome difficulty associated with abrasive water jets, which presently limits their usefulness, is wear and erosion of the mixing tube walls. Since the water jet's speed ranges between 100-500 m/sec, and the abrasive particle size can be as high as 40% of the mixing tube's diameter, the mixing tubes must be replaced frequently, sometimes only a matter of hours.
Additionally, the wear of the mixing tube walls leads to the jet becoming incoherent, which causes an increase in the width of the cut (kerf) on the workpiece being cut by the jet, deterioration of cutting surface quality and loss of cutting accuracy. Hence, wear of the mixing tube walls requires constant maintenance and inspection, which leads to machine down time and increase in the operational costs of such systems.
FIG. 3 presents a schematic representation of the phenomena associated with wear of a mixing tube. Impact erosion phenomena is thought to dominate the wear in the initial portion of the mixing tube as the abrasive particles impact on the walls of the mixing tube at different impact angles. Further downstream the abrasive particles tend to travel parallel to the walls of the tube and the wear mode tends to change from impact erosion to sliding, abrasion erosion.
Present attempts to solve this wear problem include: (a) the use of mixing tubes made of very hard materials (e.g., composite tungsten carbide), (b) modifying the jet's flow structure by using an annular water jet and introducing the abrasives through a central pipe in an attempt to keep the abrasives away from the mixing tube's walls, (c) modifying the jet's flow structure by introducing the abrasives through a central pipe and having the pressurized water enter from radially inwardly directed ports whose flows combine to create a jet slurry that is focused in the mixing tube, (d) using a central deflector body prior to the mixing tube so as to create a downstream wake that helps in entraining the abrasives in the core of the water jet, (e) using abrasives that are softer than the walls of the mixing tube, and (f) attempting to configure the general shape of the mixing tube so as to minimize its wear.
All of the presently available techniques to reduce mixing tube wear have major deficiencies. The very hard materials used for mixing tubes are expensive. Modification to the jet flow structure by introducing secondary flow phenomena is useful only with relatively slow flows and small abrasive particles; such modification also causes jet expansion and secondary flow phenomena that limit one's capability to control the cutting process. The use of abrasive particles softer than the mixing tube's walls reduces cutting efficiency.
Thus, despite extensive development efforts to reduce wear in the mixing tube of a cutting jet, there exists a continuing need for further improvements in this area. The present invention provides such an improvement.